Sediment exchange between the beach and the inner shelf

Abstract

Embayed beaches are often considered closed sediment cells. Contrarily to this widely spread idea, recent studies suggest that the inability of certain embayments to recover to storms is a consequence of significant sediment exchange between the beach and neighbouring areas during extreme wave events; however, the physical coupling is still poorly understood. The estimation of the depth of closure in relation to the depth in front of the bounding headlands along embayed coastlines allows questioning whether embayments experience more headland bypassing than expected. The macrotidal, embayed and high-energy coastline of SW England was used as a natural field laboratory to identify the ‘active’ nearshore limits (Depth of Closure, DoC, and Depth of Transport, DoT) using a multi-method approach that includes observations of shoreface morphology and sedimentology, offshore/inshore wave formulations and bed shear stress computations. Values of DoC are c. 10 – 15 m; and the computed DoT, represented by the upper-plane bed transition attained under extreme conditions, exceeds 30 m depth in the study area. Even though many headlands appear sufficiently prominent to suggest a closed boundary, significant wave- and tide-driven sediment transport is likely to occur beyond the headland base during extreme events. DoT was computed across a broad wave-current parameter space, further highlighting that tidal currents can increase this closure depth estimate by ~10 m along macrotidal coastlines, representing a 30% increase compared to tideless settings. A combination of LiDAR, UAV photogrammetry, RTK-GNSS, single-beam and multi-beam echosounder surveys, that encompassed the dune system to > 40 m water depth of Perranporth embayment was used to quantify the sediment budget. Inter-annual dynamics and embayment sub-systems response over a 10-year period that included extreme storm erosion and post-storm recovery were evaluated, demonstrating that Perranporth is neither closed, nor balanced. The very significant net changes, representing a loss of c. 100 m3 m-1 during the extreme storm epoch (2011 – 2016, period encompassing 2013/14 storms) and a gain of c. 200 m3 m-1 during the subsequent recovery period (2016 – 2018), indicated that significant sediment transport occurred seaward of the base of the headlands and beyond the morphological depth of closure. It is further demonstrated that the inter-tidal region is partly uncoupled from the sub-tidal region, with the former region dominated by cross-shore sediment fluxes, whereas the sub-tidal region is also significantly affected by longshore sediment fluxes. The nearshore sediment transport dynamics along a 15-km stretch of coastline encompassing Perranporth beach were investigated using Delft3D. Numerically-modelled wave-driven and tidal currents were used to support interpretation of sediment flux pathways inferred from the morphological observations of Chapter 3. Multi-embayment circulation, mega-rip formation where an alongshore current is deflected offshore (0.7 m s-1 at > 20 m depth) in the down-wave sectors and cross-shore exchanges extending to depths that exceed the base of the headlands (c. 104 m3 day-1) dominated during extreme events. Accretionary phases over moderate-high swell periods were associated with clockwise intra-embayment circulation with predicted currents inducing redistribution in the long embayments (> 103 m3 day-1) towards the south. This circulation mode is combined with significant bypassing rates around the shallower and wider headlands (102 – 103 m3 day-1). A simple empirical parameterisation for sediment bypass based on offshore wave-conditions (r > 0.92) is presented, allowing prediction of sediment fluxes on the lower shoreface and sediment budgets over multi-annual time scales. This work provides new insights on nearshore sediment dynamics at different spatial and temporal scales, with a major focus on headland and cross-embayment bypass. This thesis demonstrates that headland bypassing is more widespread than commonly assumed, leading to a shift in understanding of sediment budgets along exposed and macrotidal embayments, particularly along sediment starved coastlines, while contributing to the knowledge of the processes affecting coastal vulnerability and long-term evolution of embayed beaches.